As oil exploration continues in remote locations, the use of offshore drilling techniques and structures will become more commonplace in ice-infested areas. Platforms are continually erected in isolated areas that have extremely severe weather conditions. However, the structures that operate in more temperate climates cannot usually be employed here because they must be able to cope, not only with severe arctic storms and sea ice incursions, but also with large and small icebergs that are driven by wind, current and wave action. Because of these conditions, many different types of platform designs have arisen in an attempt to cope with the harsh weather and other natural elements.
Currently, much exploration is conducted in the arctic and in the ice-infested waters off Alaska, Canada, and Greenland. To cope with the iceberg and weather problem, some structures attempt to resist these large ice masses by simply being large enough to withstand the forces from the ice features. Examples of these designs may be seen in dual cone structures, such as U.S. Pat. No. 4,245,929, large reef-like structures, or many other gravity based large concrete-steel configurations, see also U.S. Pat. No. 4,504,172. However, these structures are usually permanently affixed to the bottom. As such, they do not lend themselves to either reuse or quick site evacuation.
Another design is a tension-leg platform (TLP) with disengageable or extensible legs as described in U.S. Pat. Nos. 3,955,521 and 4,423,985. These too have their inadequacies. The TLP cannot take a substantial deck load. Furthermore, there may be problems with icebergs that have drafts large enough to scour the sea floor. Most TLP structures have exposed wellheads and anchoring systems and thus would incur substantial damage if an iceberg of this size came along. Additionally, since the platform is naturally buoyant, the tendons are under constant tension which generally shortens the life of the tie down system.
Another factor to be considered is cost. Generally, the type of large ground based structure that may be used for arctic exploration and production is very expensive and time consuming to build. With the unproven nature of some of the oil prospects, the harshness of the environment, the increased costs due to the weather down time, the probability of failure, and even the political climate, it becomes even more risky for an oil company to invest a large amount of money or time. In the event of an accident or other type of misadventure, losses could be greatly multiplied.
To overcome many of the disadvantages of these previously discussed arctic structures, it would be advantageous to combine some of the principles of the gravity-based structures with those of the floating structures. This is accomplished by constructing a platform that has subsurface hull chambers that may alternatively provide buoyancy and ballast. This structure may be floated to a drilling or production site and slowly filled with ballast until it rests on the sea floor. When a situation, threatening to the structure, presents itself, the platform may be deballasted and removed from the site. However, this deballasting procedure is quite slow (on the order of 6 to 7 hours) and since it will possibly be done in rough seas, there is a chance that the structure may be damaged when it "bounces around" during its slow ascent.
To minimize this problem, a sudden and rapid ascent is required. This means that it is necessary to keep the platform on location down with a hold-down means while it is deballasting. Once it has been fully deballasted, the hold-down means may then be released to allow the platform to quickly ascend to the surface and escape damage.
This hold-down system may be mechanical or hydraulic, however, because a mechanical system will be subject to a high degree of tension just prior to release, and may not assure a simultaneous release of all mechanical systems, a hydrostatic sealing system is chosen. This hydrostatic system will hold the structure to the base from the beginning of the deballasting procedure to the time when deballasting is complete. When this occurs, the structure may be quickly detached by releasing the seal and then floated away from the impending danger.
To eliminate most of the problems of these previously-mentioned arctic structures for use in ice-infested waters, the Removable Bottom Founded Structure (RBFS) was developed to provide a platform which may be removably detached from its base with the help of the aforementioned seals and, if necessary, transported to a safer location.